1. Field of the Invention
The present invention relates to a liquid crystal display device used for a display of a clock, a calculator, a word processor, a small TV set, etc., and a liquid crystal compound and a liquid crystal composition (in particular, a ferroelectric liquid crystal composition) preferably used for the liquid crystal display device.
2. Description of the Related Art
At present, most of the above-mentioned liquid crystal display device s use a TN (twisted nematic) display system. TN liquid crystal display devices, which utilize a nematic liquid crystal composition, are roughly classified into two groups.
One group includes liquid crystal display devices which utilize an active matrix system in which a switching element, e.g., TFT (thin film transistor), is provided at each pixel. The active matrix system realizes a display quality comparable to that of a CRT (cathode ray tube). However, due to the complicated structure, it is costly to produce a large active matrix liquid crystal display device.
The other group includes liquid crystal display devices which utilize an STN (super twisted nematic) system. The STN system realizes an improved contrast and viewing angle characteristic, compared with those of a conventional simple matrix system. However, the STN system is not suitable for displaying animations because of its slow response speed. Furthermore, the STN system has the disadvantage of a lower display quality than that of the active matrix system using TFTs. On the other hand, the STN system can be produced at low cost. Considering a display quality and a production cost, the active matrix system and the STN system respectively have both advantages and disadvantages.
An example of a display system overcoming the above-mentioned problems includes a liquid crystal display system using ferroelectric liquid crystal (FLC) which is smectic liquid crystal exhibiting spontaneous polarization (R. B. Meyer et al., Journal de physique, 36L-69 (1975)). When the ferroelectric liquid crystal is placed between a pair of substrates (cell) opposing each other with a gap of several .mu.m therebetween, the helical structure of the liquid crystal is eliminated, so that bistable switching having a memory effect can be realized. Such ferroelectric liquid crystal is called surface stabilized ferroelectric liquid crystal (SSFLC). Currently, ferroelectric liquid crystal refers only to SSFLC. SSFLC was proposed by N. A. Clark, S. T. Lagerwall, Appl. Phys. Lett., 36, 899 (1980).
The SSFLC system has a high response and a memory property 100 to 1000 times as high as those of the conventional liquid crystal display device using nematic liquid crystal, and furthermore, has a wide viewing angle characteristic. These properties suggest the usefulness of SSFLC for a large capacity display.
However, as the study proceeds, problems of SSFLC which should be overcome are becoming apparent. Among them, it is the first objective to obtain a stable memory property. The reason why it is difficult to obtain a stable memory property in SSFLC lies in nonuniform smectic layer structure (e.g., twisted alignment, a chevron structure, etc.), and the generation of an inner reverse electric field which is considered to be caused by the excessively high spontaneous polarization.
As one means for obtaining a stable memory property, an AC stabilizing effect using a ferroelectric liquid crystal composition having a negative dielectric anisotropy (hereinafter, referred to as ".DELTA..epsilon.") has been proposed (Paris Liquid Crystal Conference, p. 217 (1984)). Hereinafter, the AC stabilizing effect will be described.
Liquid crystal molecules having a negative .DELTA..epsilon. have the property that they are aligned parallel to substrates (i.e. molecule long axes are aligned vertically to the direction of an electric field) while an electric field is being applied in a direction vertical to electrodes provided on the substrates in a cell subjected to a homogeneous alignment treatment. In the case where a low frequency electric field is applied, spontaneous polarization of the liquid crystal molecules can follow the inversion of the electric field. Therefore, the liquid crystal molecules move to another stable state along with the inversion of direction of the electric field and become parallel to the substrates due to the effect of .DELTA..epsilon.. In contrast, in the case where a high frequency electric field is applied, spontaneous polarization of the liquid crystal molecules does not follow the inversion of the electric field. Therefore, only .DELTA..epsilon. exerts its effect, and the liquid crystal molecules do not move while the direction of the electric field is inverted. Thus, the liquid crystal molecules remain parallel to the substrates. This is a mechanism of obtaining a memory property utilizing the AC stabilizing effect, which enables a high contrast to be obtained. An example of the AC stabilizing effect has already been reported in SID'85 Digest, p. 128 (1985).
Regarding a method for utilizing a liquid crystal material having a negative .DELTA..epsilon., a .tau.-V.sub.min driving method has been proposed by Surguy et al. (P.W.H. Ferroelectrics, 122, 63 (1991)). This is a promising method for realizing a high contrast, and P. W. Ross, SID'92, 217 (1992) discloses a ferroelectric liquid crystal display using this method. Hereinafter, the .tau.-V.sub.min driving method will be described.
In the case of ferroelectric liquid crystal not having a negative .DELTA..epsilon., a pulse width required for memory monotonously decreases with the increase in a driving voltage (V). On the other hand, in the case of ferroelectric liquid crystal having a negative .DELTA..epsilon., a voltage (V)-memory pulse width (.tau.) characteristic has a local minimum (.tau.-V.sub.min), as shown in FIG. 9. Surguy et al. has reported a JOERS/Alvey driving method using this characteristic. The principle of this driving method is as follows: as shown in FIG. 10, assuming that V.sub.s is a scanning voltage and V.sub.d is a data voltage, a memory state of a ferroelectric liquid crystal device is switched when a voltage .vertline.V.sub.s -V.sub.d .vertline. is applied, and the memory state is not switched when a voltage .vertline.V.sub.s -V.sub.d .vertline. which is higher than the voltage .vertline.V.sub.s -V.sub.d .vertline., or a voltage .vertline.V.sub.d .vertline. which is lower than the .vertline.V.sub.s -V.sub.d .vertline., is applied.
As described above, the ferroelectric liquid crystal material having a negative .DELTA..epsilon. can utilize the AC stabilizing effect and the .tau.-V.sub.min driving method. Therefore, there may be a possibility that the ferroelectric liquid crystal material having a negative .DELTA..epsilon. can be used to realize a ferroelectric liquid crystal display device with a high contrast and a high-speed switching.
In the case where the above-mentioned ferroelectric liquid crystal material is actually used for a liquid crystal display device, a number of characteristics are required. However, at present, it is difficult to satisfy all the requirements by a single compound. Thus, generally, optimization of characteristics is conducted by mixing various compounds. For example, in the case of a ferroelectric liquid crystal material having a negative .DELTA..epsilon. which has a low viscosity and a high-speed response, it requires a high driving voltage. The driving voltage can be decreased by mixing a material having a negative .DELTA..epsilon. with a large absolute value therewith.
Hithertofore, some liquid crystal compounds have been reported. For example, Japanese Laid-open Publication No. 60-54375 discloses the following compound: ##STR3## wherein either one of A and B is an alkyl group or an alkoxy group having 1 to 15 carbon atoms, and the other is a chlorine atom or a fluorine atom.
Furthermore, J. Barbera et al., LIQUID CRYSTALS, Vol. 11, No. 6 (1992) 887-897., C. G. Seguel et al., LIQUID CRYSTALS, Vol. 11, No. 6 (1992) 899-903, and J. Bartulin et al., Mol. Cryst. Liq. Cryst., Vol. 225 (1993) 175-182 discloses the following compound: ##STR4## wherein C and D are alkoxy groups.
The compound represented by the above Formula (i) is nematic liquid crystal having a positive .DELTA..epsilon. and does not exhibit a smectic phase. Thus, the compound (i) is not suitable as a ferroelectric liquid crystal material. On the other hand, the compound represented by the above Formula (ii) exhibits a smectic phase and is suitable as a ferroelectric liquid crystal material. However, the compound (ii) also has the disadvantage that it has a high viscosity due to two alkoxy groups on C and D.
Irrespective of whether or not the above-mentioned .tau.-V.sub.min driving method is used, in order to realize a high switching in ferroelectric liquid crystal, a material having large spontaneous polarization and a low viscosity is generally required. This is represented by the following Formula: EQU .tau..eta./P.sub.s
wherein .tau. is a response time or a pulse width required for memory; .eta. is a viscosity; and P.sub.s is spontaneous polarization.
However, according to the .tau.-V.sub.min driving method, the following relationship holds between a voltage V.sub.min and P.sub.s where .tau. becomes a local minimum .tau..sub.min : ##EQU1## wherein d is a cell thickness; .epsilon..sub.0 is a dielectric constant in a vacuum; Ae is a dielectric anisotropy; and .theta. is a tilt angle. As is understood from this formula, when the spontaneous polarization P.sub.s is increased in order to realize a high-speed switching, V.sub.min is also increased. This results in a high driving voltage and an increase in power consumption.
Accordingly, in order to realize high-speed driving while retaining V.sub.min at a low level, a ferroelectric liquid crystal material having a negative .DELTA..epsilon. with a large absolute value and a low viscosity is required. In order to obtain a stable memory property using the above-mentioned AC stabilizing effect, a ferroelectric liquid crystal material having a negative .DELTA..epsilon. with a large absolute value is also required.
However, the compound (i) is nematic liquid crystal having a positive .DELTA..epsilon., so that it cannot be used as a ferroelectric liquid crystal material having a negative .DELTA..epsilon.. Furthermore, the compound (ii) has a high viscosity (i.e., is likely to have a high phase transition temperature), so that a driving voltage cannot be decreased.